Prior Art Statement
The following publications are representative of the most relevant prior art known to the applicant at the time of filing the application:
U.S. Pat. Nos. 2,765,512, Oct. 9, 1956, R. A. Nesbit; 2,942,991 June 28, 1980, E. Smith; 2,964,823 Dec. 20, 1960, J. I. Fredriksson; 3,718,564 Feb. 27, 1963 J. A. C. Ebrey et al; 3,882,010 May 6, 1975 E. J. Szabo; 4,121,987 Oct. 24, 1978 William Ryan et al.
FOREIGN PATENT APPLICATION NO. 2,003,183A, Mar. 7, 1979, United Kingdom.
Other Publications
"`Elephant` modernizes Whiteware Process", pp. 30-32 and 44, Ceramic Industry, May, 1980.
F. S. Entelis et al, "Design of Cathodes for Electrophoretic Forming of Porcelain Cups"; Science For The Ceramic Industry, Volume 36, July 1980, pp. 683-685 (translated from Steklo i Keramika, No. 12, pp. 11-12, December 1979), Glass and Ceramics.
U.S. Pat. No. 2,942,991 to E. Smith discloses a basic casting process wherein an aqueous slip is cast into a conventional porous mold. The reference is relevant for its statement at lines 19-21 column 3, that the viscosity or the slurry used in the process is not critical and can be within a wide viscosity range. This was, at the time and reference issued, the belief of those skilled in the art; that is still the belief at the present time.
J. J. Fredriksson discloses the importance of a bimodal particle size distribution of the refractory particles contained in the slip as utilized in the more conventional porous mold slip casting process. He discovered that porosity, i.e. density, of the piece being cast, can be controlled by using a slip made up of about 50% of particles in the 0.1 to 8 micron range and 50% of particles in the 45 to 150 microns.
The Ebrey et al patent teaches an electrophoretic slip casting method for the casting of ceramic articles such as pottery, by combining conventional slip casting with electrophoresis. Ebrey et al utilize a conventional type porous plaster of paris mold which is then provided with a conductive coating of a low melting metal on the outside of said porous mold. A clay slip is cast into the porous mold and a short period of time is allowed to lapse as an initial cast-up period of e.g. 2-4 minutes. After this initial cast-up period, a metal electrode is immersed in the slip and a potential of 200-300 volts is applied; the metal of the electrode is not critical but is most desirably one that does not electrolyze too readily. Preferred electrode metals are alloys of tin, zinc and bismuth. Surplus slip and water is decanted and the green casting is partially dried and removed from the mold.
Another variant on the process of electrophoretic casting of inorganic materials, i.e. refractory particles, is that disclosed by Szabo. The reference recognizes the problem of gas evolution at the depository electrode when that electrode is, for example, a metal coating. The evolution of gas caused by electrolysis, results in cavities or holes in the cast shape. Szabo solves this problem by replacing the prior art metal coating on the mold with a porous conductive coating of graphite and powdered refractory pig graphite and alumina or silica. The pores of this coating allow the gas bubbles to migrate away from the refractory material being electrophoretically deposited using a potential of 1-10 volts/cm. The mold form on which the graphite-refractory coating is applied, is made of wax or a thermoplastic polymer. The deposited refractory shape is dried and the wax (or plastic) mold melted away freeing the casting.
The Ryan et al reference is also primarily concerned with the gases generated during electrophoretic casting of a ceramic or refractory aqueous slip and solves the problem with a porous depository mold. In a preferred embodiment the conductive casting mold, in its entirety, is porous and is fabricated from a mixture of powdered carbon and particulate inorganic material like clay, silicon carbide, cement, aluminum phosphate, thermosetting resin, and the like. In the alternative, the main body of the mold may be constructed of plastic with a porous, carbonaceous conducting surface or coating, only on the operative surface of the mold, i.e. the surface on which the refractory slip will be deposited. Ryan et al. recognize that the size of the pores in the conductive mold are critical if optimum removal of generated gases is to be accomplished. Accordingly, the particle size of the powdered materials used to form the conductive mold are carefully selected and formed so as to produce a mold with the desired porosity. This is accomplished by using only graphite and other materials that have a maximum particle size of from 70 to 200 microns. The other electrode may be a metal such as zinc or may be a carbon based material similar to that of the depository mold. Ryan et al use an anode-cathode potential of about 50 to 80 volts, but recognize that lower or higher potentials may be used depending on the size of the casting being formed. The method usually involves a drying step subsequent to the casting phase of the process and prior to firing.
The British application No. 2,003,183A discloses electrophoretic slip casting of ceramic parts by applying a potential to a slip of ceramic powder by way of a metal container holding the slip functioning as one electrode and a mandrel functioning as the depository electrode. The potential is applied and the mandrel is preferably rotated, particularly if tubes are being produced; the mandrel can, however, be any shape desired. According to the reference, the porosity of the cast pieces can be varied by varying the particle size of the ceramic material in the slip. After completion of the casting step, the green casting is dried, isostatically pressed and the mandrel removed. The final step in the process is firing of the pressed, green shape. These parts are fairly thin walled, in the order of 5 mm.
The Ceramic Industry, May 1980 article is a description of a commercial electrophoretic process for the production of whiteware. It is relevant for its teaching of the use of a potential of 23 volts, zinc coated electrodes and the fact that the cast product is still flexible and contains 10-18% by weight of water. This equipment produces only ribbon or plate stock.
F. S. Entelis et al. disclose an advancement in the electrophoretic casting art whereby the cathode, the non-depository electrode, is one or more metallic strips bent in specific configurations to create a uniform distribution of potential gradients in the system thus providing optimum deposition of the slip particles on the depository electrode i.e. the anode. It was also found by these workers that either the anode or cathode must be rotated. The full outer surface of the shape being formed is defined by the entire anode. The major distinction between the teachings of Entelis et al and the present invention is that in the latter the anode, that is the depository electrode, makes up substantially less than the complete surface of the shape being cast; thereby greatly reducing the interactions between slip, electrodes, and applied voltage which cause hydrolysis. The use of partial electrodes reduces the requirements of uniform fields to obtain uniform thicknesses in that additional surfaces (including those of non-uniform wall thickness which cannot be controllably produced by the referenced technique) can be defined by electrically inactive surfaces.
The Nesbit reference teaches freezing of a green slip cast article to facilitate removal from the mold. This well known method can be employed in the process of the present invention for the same purpose, and may even be modified by vacuum removal of the residual water prior to thawing.
In all cases, the voltage utilized throughout the prior art, has been straight or pure DC voltage. The present invention utilizes AC signals imposed on DC potential reference voltages as well as straight DC potential.